The present invention relates to a method of forming a stack of sheets on a delivery surface which includes the steps of depositing a sheet and, with the use of a pusher element, pushing against an edge of the sheet in order to apply the sheet against a stop. The present invention also relates to a device for forming a stack of sheets using the present method said device including a delivery surface and a stop and a pusher element movable in the direction of the stop.
A method and device according to U.S. Pat. No. 5,054,766 is known wherein sheets are collected and arranged into a straight stack for after-treatment, for example, stapling, punching or stitching. A known application is the collection of printed sheets in a copying machine or printer. The collecting stations associated with such machines must be able to operate reliably at the speed of the copying machine or printer, which may be very high, for example more than 60 or even more than 100 prints per minute on A4 format. In addition, it must be possible to be able to process an ever-increasing variety of materials. The variety of materials varies from various plastic films, very thin and flimsy papers, to thick stiff papers. The gram weights of the types of paper that can be processed to modern requirements in modern printers and copying machines vary between 60 to 65 g/m2 and 250 to 300 g/m2. A problem that occurs with the various types of paper in use is that the deposited printed sheets do not lie absolutely flat, but may have an upwardly or downwardly directed curl, or in the case of very thin sheets may simply hang down limply. The pusher element that has to move the sheets against a stop in order to form a straight stack does not act on the front edge in such cases, but on the curled or limply hanging part of the sheet. The result is that the sheet is mainly bent, and is hardly moved, if at all, and is not straightened against the stop.